1. Field of the Invention
The present invention relates to a grating and a method and apparatus for forming the grating, and more particularly, to the grating that is formed in the core locating in the center of the optical waveguide and the method and apparatus for forming such a grating in the above-mentioned core.
2. Description of the Related Art
In general, a grating, which is formed in a core of an optical waveguide, may be classified into a Bragg grating and a long-period grating. The term xe2x80x9coptical waveguidexe2x80x9d in the following description includes an optical fiber.
An outline of these gratings will be described. The Bragg grating has a grating period, which is substantially the same as the wavelength of light used in an optical communication utilizing the above-mentioned optical waveguide. The Bragg grating serves as a mirror that reflects only the light having the specific wavelength, which satisfies the Bragg conditions relative to the grating period.
There occurs no propagation of the light reflected by the mirror to the downstream side of the grating in the travelling direction of the light. Accordingly, the Bragg grating may be used as an optical filter for blocking the light having a prescribed wavelength.
Expansion or contraction of a portion of the optical waveguide including the above-mentioned grating, due to heat and/or external vibration, causes its grating period to expand or contract simultaneously. Consequently, there occurs change in wavelength of light, which satisfies the Bragg conditions. The Bragg grating may be used as a temperature sensor or a vibration sensor utilizing the above-mentioned change detected.
On the contrary, the other kind of grating, i.e., the long-period grating has a grating period, which is extremely longer than the wavelength of light used in the optical communication utilizing the optical waveguide, unlike the Bragg grating. Consequently, the long-period grating provides coupling of certain two optical propagation modes. The long-period grating may therefore be used as a mode coupler.
In case where the optical propagation mode of light having a certain wavelength and a cladding mode (i.e., a mode in which light is irradiated outside the optical waveguide) are coupled to each other by the above-mentioned long-period grating, the long-period grating may be used as an optical filter for irradiating light having the certain wavelength outside the optical waveguide.
In addition, the long-period grating may also be used as a temperature sensor or a vibration sensor in the same manner as in the Bragg grating.
With respect to a formation method of the grating in the core, there has conventionally been adopt a method of irradiating a laser beam to the core so as to form the grating utilizing refractive index-variation of the core due to the irradiation of the laser beam.
However, in the conventional method of irradiating the laser beam to the core to form the refractive index-variation portions of which the grating is formed, it is impossible to form any grating in an optical fiber having a core formed of material in which refractive index-variation does not occur even when the laser beam is irradiated, thus causing a problem.
The above-mentioned problem is serious in view of the fact that material for forming the optical fiber mostly has a low photosensitivity to light.
When the grating is formed by irradiating the laser beam to the optical fiber having the core, which is formed of material having a high photosensitivity to the laser beam, in the conventional manner, it is necessary to apply hydrogen loading to the optical fiber, for example, at room temperature under high pressure for a period of time of from 10 days to two weeks, in order to increase the photosensitivity of the core. The formation process therefore becomes complex and the cost of the product increases, thus causing problems.
In addition, in the conventional formation method, only small variation in refractive index occurs in practice in the refractive index-variation portions formed, due to the low photosensitivity of material for the core to the laser beam, thus making it hard to obtain the grating in an effective manner.
The low photosensitivity of the material for the core to the laser beam requires maintenance of a high condensing property and a high irradiation power during the irradiation of the laser beam. The conventional method is therefore not suitable for mass production of the grating.
In the conventional method, formation of a plurality of point defects in the core by the laser beam irradiation causes the refractive index to change, thus shortening the service life of the grating.
An object of the present invention, which was made to solve the above-described problems, is therefore to provide a grating, which is formed in a core of an optical waveguide including an optical fiber and a high efficiency and a long service life, as well as a method and apparatus for forming the grating in an easy and effective manner.
In order to attain the aforementioned object, a grating formed in a core of an optical waveguide comprises a plurality of refractive index-variation portions that are formed in said core by implanting accelerated ions into said core.
The grating is composed of the plurality of refractive index-variation portions, which are formed by implanting ions into the core. It is therefore possible to increase refractive index variation rate in the ion-implanted portions. A high efficiency of the grating can be provided by improving optical reflectance or coupling efficiency between modes in the refractive index-variation portions.
Change in refractive index utilizing the ion implantation is principally caused by densification induced by the ion implantation in material for the optical waveguide. The induced densification is stable relative to heat in comparison with the point defects that are induced by the laser beam. It is therefore possible to increase the service life of the grating and impart a high temperature resistance to it.
The refractive index-variation portions may be spaced apart from each other in a direction of the central axis of said optical waveguide and the spacing and the number of said refractive index-variation portions may correspond to properties, which are to be imparted to said grating.
According to such a feature, it is possible to obtain the grating having a high efficiency with a simple structure.
Each of said plurality of refractive index-variation portions may be formed in a plane perpendicular to the central axis of said optical waveguide in said core.
According to such a feature, it is possible to obtain the grating having a high efficiency with a simple structure.
The grating may be a Bragg grating. In this case, it is possible to improve the efficiency as the Bragg grating.
The grating may be a long-period grating. In this case, it is possible to improve the efficiency as the long-period grating.
A method of the present invention for forming a grating in a core of an optical wave guide, comprises:
an acceleration step for accelerating ions to provide accelerated ions;
a passing step for causing said accelerated ions to pass through a mask having a shape corresponding to said grating to be formed; and
an implantation step for implanting the ions, which have passed through said mask, into said core, to form a plurality of refractive index-variation portions for said grating in said core.
In the above-mentioned method, the grating is composed of the plurality of refractive index-variation portions, which are formed by implanting the accelerated ions into the core. It is therefore possible to form effectively the grating even in the optical waveguide having the core, which has a low photosensitivity to the laser beam or no photosensitivity to it.
The grating is composed of the plurality of refractive index-variation portions where the refractive index variation rate is increased. As a result, a high efficiency of the grating can be provided by improving optical reflectance or coupling efficiency between modes in the refractive index-variation portions. In addition, it is possible to increase the service life of the grating and impart a high temperature resistance to it.
Another method of the present invention for forming a grating in a core of an optical wave guide, comprises:
an acceleration step for accelerating ions to provide accelerated ions;
a convergence step for converging an ion beam, which is formed of said accelerated ions, to make a beam diameter of said ion beam identical with or smaller than a width of refractive index-variation portions for said grating in a direction of the central axis of said optical waveguide so as to provide a converged ion beam; and
an implantation step for intermittently irradiating said converged ion beam to said core, while moving an irradiation position in the direction of said central axis, to implant said ions into said core so as to form said plurality of refractive index-variation portions in said core.
In the other method mentioned above, the grating is composed of the plurality of refractive index-variation portions, which are formed by implanting the accelerated ions into the core. It is therefore possible to form effectively the grating even in the optical waveguide having the core, which has a low photosensitivity to the laser beam or no photosensitivity to it.
The grating is composed of the plurality of refractive index-variation portions where the refractive index variation rate is increased. As a result, a high efficiency of the grating can be provided by improving optical reflectance or coupling efficiency between modes in the refractive index-variation portions. In addition, it is possible to increase the service life of the grating and impart a high temperature resistance to it.
In the above-mentioned implantation step, said ions may be implanted so that said refractive index-variation portions are spaced apart from each other in the direction of the central axis of said optical waveguide and the spacing and the number of said refractive index-variation portions corresponds to properties, which are to be imparted to said grating.
According to such a feature, it is possible to obtain the grating having a high efficiency with a simple structure.
In the above-mentioned implanting step, said ions may be implanted so that each of said plurality of refractive index-variation portions is formed in a plane perpendicular to the central axis of said optical waveguide in said core.
According to such a feature, it is possible to obtain the grating having a high efficiency with a simple structure.
In the above-mentioned method of the present invention, the grating to be formed may be a Bragg grating. In this case, it is possible to form the Bragg grating having high efficiency even in the core formed of any material.
In the above-mentioned method of the present invention, the grating to be formed may be a long-period grating. In this case, it is possible to form the long-period grating having high efficiency even in the core formed of any material.
In the above-mentioned method of the present invention, the ions to be implanted may comprise protons.
With respect to this feature, it is necessary to cause the ions to pass through the cladding of the optical waveguide to reach the core, in order to form the refractive index-variation portions in the core by means of ion implantation. Protons have a penetration depth, by which the ions penetrate into material during the ion implantation step, which depth is largest among all the ions under the constant accelerated energy. It is therefore possible to cause the ions to reach the core by small acceleration energy, thus permitting to form effectively the grating.
In the above-mentioned method of the present invention, the ions to be implanted may comprise helium ions.
With respect to this feature, the helium ions have the next largest penetration depth to that of protons. It is therefore possible to cause the ions to reach the core by small acceleration energy, thus permitting to form effectively the grating.
Helium ion has a weight, which is four times as much as the weight of proton. Consequently, variation in refractive index caused during the helium ion implantation step is about four times as much as that in refractive index caused during the proton implantation step. It is therefore possible to obtain the refractive index-variation portions having a prescribed rate in change in refractive index by a small amount of ions implanted, thus permitting to effectively form the grating.
In addition, helium ion has an excessively low reactivity. It is therefore possible to form the refractive index-variation portions without causing degradation of the core.
An apparatus of the present invention for forming a grating in a core of an optical wave guide, comprises:
an acceleration device such as an accelerating electrode for accelerating ions to provide accelerated ions;
a passing device such as a scanning device for causing said accelerated ions to pass through a mask having a shape corresponding to said grating to be formed; and
an implantation device such as a mask for implanting the ions, which have passed through said mask, into said core, to form a plurality of refractive index-variation portions for said grating in said core.
In the above-mentioned apparatus of the present invention, the grating is composed of the plurality of refractive index-variation portions, which are formed by implanting the accelerated ions into the core. It is therefore possible to form effectively the grating even in the optical waveguide having the core, which has a low photosensitivity to the laser beam or no photosensitivity to it.
The grating is composed of the plurality of refractive index-variation portions where the refractive index variation rate is increased. As a result, a high efficiency of the grating can be provided by improving optical reflectance or coupling efficiency between modes in the refractive index-variation portions. In addition, it is possible to increase the service life of the grating and impart a high temperature resistance to it.
Another apparatus of the present invention for forming a grating in a core of an optical wave guide, comprises:
an acceleration device such as an accelerating electrode for accelerating ions to provide accelerated ions;
a convergence device such as a quadrupole lens for converging an ion beam, which is formed of said accelerated ions, to make a beam diameter of said ion beam identical with or smaller than a width of the refractive index-variation portions for said grating in a direction of the central axis of said optical waveguide so as to provide a converged ion beam; and
an implantation device such as a scanning electrode for intermittently irradiating said converged ion beam to said core, while moving an irradiation position in the direction of said central axis, to implant said ions into said core so as to form said plurality of refractive index-variation portions in said core.
In the other apparatus mentioned above of the present invention, the grating is composed of the plurality of refractive index-variation portions, which are formed by implanting the accelerated ions into the core. It is therefore possible to form effectively the grating even in the optical waveguide having the core, which has a low photosensitivity to the laser beam or no photosensitivity to it.
The grating is composed of the plurality of refractive index-variation portions where the refractive index variation rate is increased. As a result, a high efficiency of the grating can be provided by improving optical reflectance or coupling efficiency between modes in the refractive index-variation portions. In addition, it is possible to increase the service life of the grating and impart a high temperature resistance to it.
With respect to the above-mentioned apparatus of the present invention, in said implantation device, said ions may be implanted so that said refractive index-variation portions are spaced apart from each other in the direction of the central axis of said optical waveguide and the spacing and the number of said refractive index-variation portions corresponds to properties, which are to be imparted to said grating.
According to such a feature, it is possible to obtain the grating having a high efficiency with a simple structure.
With respect to the above-mentioned apparatus of the present invention, in said implantation device, said ions may be implanted so that each of said plurality of refractive index-variation portions is formed in a plane perpendicular to the central axis of said optical waveguide in said core.
According to such a feature, it is possible to obtain the grating having a high efficiency and a simpler structure.
In the above-mentioned apparatus of the present invention, the grating to be formed is a Bragg grating. In this case, it is possible to form the Bragg grating having high efficiency even in the core formed of any material.
In the above-mentioned apparatus of the present invention, the grating to be formed is a long-period grating. In this case, it is possible to form the long-period grating having high efficiency even in the core formed of any material.
In the above-mentioned apparatus of the present invention, the ions to be implanted may comprise protons.
With respect to this feature, it is necessary to cause the ions to pass through the cladding of the optical waveguide to reach the core, in order to form the refractive index-variation portions in the core by means of ion implantation. Protons have a penetration depth, by which the ions penetrate into material during the ion implantation step, which depth is largest among all the ions under the constant accelerated energy. It is therefore possible to cause the ions to reach the core by small acceleration energy, thus permitting to form effectively the grating.
In the above-mentioned apparatus of the present invention, the ions to be implanted may comprise helium ions.
With respect to this feature, the helium ions have the next largest penetration depth to that of protons. It is therefore possible to cause the ions to reach the core by small acceleration energy, thus permitting to form effectively the grating.
Helium ion has a weight, which is four times as much as the weight of proton. Consequently, variation in refractive index caused during the helium ion implantation step is about four times as much as that in refractive index caused during the proton implantation step. It is therefore possible to obtain the refractive index-variation portions having a prescribed rate in change in refractive index by a small amount of ions implanted, thus permitting to effectively form the grating.
In addition, helium ion has an excessively low reactivity. It is therefore possible to form the refractive index-variation portions without causing degradation of the core.